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<div class="titlepage"><div><div><h4 class="title">
<a name="asio.reference.async_read.overload4"></a><a class="link" href="overload4.html" title="async_read (4 of 6 overloads)">async_read (4
        of 6 overloads)</a>
</h4></div></div></div>
<p>
          Start an asynchronous operation to read a certain amount of data from a
          stream.
        </p>
<pre class="programlisting">template&lt;
    typename <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a>,
    typename <a class="link" href="../DynamicBuffer.html" title="Dynamic buffer requirements">DynamicBuffer</a>,
    typename <a class="link" href="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</a>,
    typename <a class="link" href="../ReadHandler.html" title="Read handler requirements">ReadHandler</a>&gt;
<a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> async_read(
    AsyncReadStream &amp; s,
    DynamicBuffer &amp;&amp; buffers,
    CompletionCondition completion_condition,
    ReadHandler &amp;&amp; handler,
    typename enable_if&lt; is_dynamic_buffer&lt; typename decay&lt; DynamicBuffer &gt;::type &gt;::value &gt;::type *  = 0);
</pre>
<p>
          This function is used to asynchronously read a certain number of bytes
          of data from a stream. The function call always returns immediately. The
          asynchronous operation will continue until one of the following conditions
          is true:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
              The specified dynamic buffer sequence is full (that is, it has reached
              maximum size).
            </li>
<li class="listitem">
              The completion_condition function object returns 0.
            </li>
</ul></div>
<p>
          This operation is implemented in terms of zero or more calls to the stream's
          async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>.
          The program must ensure that the stream performs no other read operations
          (such as async_read, the stream's async_read_some function, or any other
          composed operations that perform reads) until this operation completes.
        </p>
<h6>
<a name="asio.reference.async_read.overload4.h0"></a>
          <span><a name="asio.reference.async_read.overload4.parameters"></a></span><a class="link" href="overload4.html#asio.reference.async_read.overload4.parameters">Parameters</a>
        </h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl>
<dt><span class="term">s</span></dt>
<dd><p>
                The stream from which the data is to be read. The type must support
                the AsyncReadStream concept.
              </p></dd>
<dt><span class="term">buffers</span></dt>
<dd><p>
                The dynamic buffer sequence into which the data will be read. Although
                the buffers object may be copied as necessary, ownership of the underlying
                memory blocks is retained by the caller, which must guarantee that
                they remain valid until the handler is called.
              </p></dd>
<dt><span class="term">completion_condition</span></dt>
<dd>
<p>
                The function object to be called to determine whether the read operation
                is complete. The signature of the function object must be:
</p>
<pre class="programlisting">std::size_t completion_condition(
  // Result of latest async_read_some operation.
  const asio::error_code&amp; error,

  // Number of bytes transferred so far.
  std::size_t bytes_transferred
);
</pre>
<p>
                A return value of 0 indicates that the read operation is complete.
                A non-zero return value indicates the maximum number of bytes to
                be read on the next call to the stream's async_read_some function.
              </p>
</dd>
<dt><span class="term">handler</span></dt>
<dd>
<p>
                The handler to be called when the read operation completes. Copies
                will be made of the handler as required. The function signature of
                the handler must be:
</p>
<pre class="programlisting">void handler(
  const asio::error_code&amp; error, // Result of operation.

  std::size_t bytes_transferred           // Number of bytes copied into the
                                          // buffers. If an error occurred,
                                          // this will be the  number of
                                          // bytes successfully transferred
                                          // prior to the error.
);
</pre>
<p>
                Regardless of whether the asynchronous operation completes immediately
                or not, the handler will not be invoked from within this function.
                Invocation of the handler will be performed in a manner equivalent
                to using <code class="computeroutput">asio::io_context::post()</code>.
              </p>
</dd>
</dl>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2018 Christopher M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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